1. Field of Invention
The invention relates to a fabrication method for microstructures, and in particular to a fabrication method for making microstructures with high aspect ratio using complementary metal-oxide semiconductor (CMOS) processes.
2. Related Art
Currently, there are many techniques that integrate micro-electromechanical elements and integrated circuits (IC) on a single chip. One of them is the suspending micro-electromechanical element in which a micro suspending structure is formed by etching the bottom silicon substrate of the micro-electromechanical element when the IC layout is completed.
A suspending microstructure, such as comb actuator, uses electrostatic forces to displace the suspending structure, producing the power for other micromechanical structures. Another example is the accelerometer. The external force displaces the suspending mass block and changes the charge between electrode gate plates, thereby measuring the acceleration of the object. One feature of these devices is that they are micro-electromechanical devices with high aspect ratio.
The microstructure with high aspect ratio is usually used in mechanical devices that have a large spring constant K. This is because the mechanical devices with a large spring constant K need to use the microstructures with high aspect ratio to obtain a shorter response time.
There are many methods proposed to make microstructures with high aspect ratio. For example, U.S. Pat. Nos. 5,393,375, 5,719,073, 5,846,849 and 6,051,866 disclose microstructures with high aspect ratios and manufacturing methods of the same. The main technical feature is to use thin-film deposition processes, photolithography processes, and dry etching processes to form the microstructures. The main steps are as follows. First, one or more dielectric layers are deposited as a mask layer. The photolithography process is then used to define the mask shape. Afterwards, use drying etching to form the high aspect ratio structure. Another oxide thin film is then deposited thereon. The thin film at the bottom of the grooves is removed by etching. Continuously etch the silicon substrate by dry etching and isotropic dry etching to remove the bottom of the structure so that the structure is suspended. Finally, a metal is deposited as an electrode.
In the manufacturing steps disclosed above, the stacking of different materials of thin films and further corresponding etching are only for forming a microstructure with high aspect ratio. Additional steps are needed for forming an IC layout to drive the microstructure or other circuits to connect the microstructures. Current packaging techniques are able to electrically connect the microstructure to other circuits for enabling the microstructure or measuring the desired signal. However, the integration of microstructures and other circuits has become more complicated, thus increasing cost.
On the other hand, the manufacturing methods of high aspect ratio structures described above cannot apply the processes used in semiconductor manufacturing; therefore mass production is not available. This makes product manufacturing a big problem.